I noticed that not a lot of authors of math books don't use the $\leadsto$ to denote the next step. As far as I understand it, that is supposed to mean "the next step of the proof is". Instead, I notice that they use $=$ to mean the same thing, or also $\implies$ or $\iff$. Which symbol is the most appropriate for stating the next step? Can we use $\implies$ and $\leadsto$ interchangeably? Is this accepted in formal proofs?
Mathematics is about understanding, not loads of symbols. Use words. Most often just placing one equation after the other, perhaps with a few well-placed comments interspersed, is much easier to read.
Don't introduce new notation if you can help it. Yes, it might sometimes be worthwhile to introduce new notation, for a rather complex concept that repeats often enough (e.g. Landau's $O()$, later extended by Knuth to $\Omega()$, $\Theta()$, and so on). "Next step is..." just doesn't cut it.